For a paint used to exterior walls and signboards in building materials, and vehicles and the like, not only color vividness and designability, but also light resistance against degradation by sunlight irradiation and durability against environmental change associated with weather changes are required. Thus, a substance that protects a coated body from ultraviolet rays and the like is used to protect components contained in a paint and a coating film, in a method of mixing it in the paint or in a method of coating it on the coating film.
Generally, use of a metal oxide as a material for protecting a coated body from ultraviolet rays and the like is effective for such paint. When the metal oxide is an iron oxide, it is required to reduce the influence of visible light, in order not to spoil protective ability from ultraviolet rays and the like for the coated body, as well as color characteristics such as a tint generated from the coated body, its chroma, transparency, and designability of a product. In particular, it is required that an iron oxide used in a red-colored paint transmits only red light, and absorbs visible lights other than red light as much as possible, for example, in case of identifying the color by lights passing through the coating film in a coated body.
As of an iron oxide for protecting a coated body from the ultraviolet rays and the like, Patent Literature 1 discloses a coloring pigment for sunlight high reflecting coating, comprising red iron oxide or yellow hydrous iron oxide having an average particle diameter of 10 nm to 300 nm. Patent Literature 2 discloses an iron oxide as a needle-shaped silica-coated Bengara red pigment having an average length of 500 nm and an average diameter of 100 nm. The iron oxide described in Patent Literature 1 or 2 may be used by mixing with the paint described in Patent Literature 3 or 4.
In a highly designed laminated coating film as described in Patent Literature 3 or Patent Literature 4 and a laminated coating film, by increasing difference between highlight (brightness, vividness) and shade (darkness) for a particular color when a light shines on a coated body, intensity of the reflected light varies greatly depending on the observation angle, to realize depth feeling, dense feeling and strong shadow sense (contrast of highlight and shade). Therefore, for a coating film comprising a coloring material such as an iron oxide, it is required to enhance transmittance for a particular color in order to enhance the highlight, and to reduce reflectivity for a particular color in order to increase difference between highlight and shade.
However, the iron oxide or the silica-coated iron oxide described in Patent Literature 1 and Patent Literature 2, have high average reflectivity in the visible region, especially in the range of the wavelength of 620 to 750 nm effective to a red-colored coated body. When used for the coating film or the coated body described in Patent Literatures 3 and 4, difference between highlight and shade is reduced, and the coating film or the coated body looks blurred, and especially color characteristics of a red paint or a red coating film, and designability of a coating film product are impaired. Thus, protection from ultraviolet rays and the like and transparency were incompatible. Further in the case of using the iron oxide particles in a paint and a coated body, the color characteristics of the iron oxide particles themselves are as important as those of the coloring material contained in the paint and the coated body. However, Patent Literature 2 describes the silica coating for inhibition of photocatalytic activity in the described silica-coated iron oxide, but does not describe specific silica coating for controlling color characteristics.
Further, Patent Literature 5 describes a black pigment of a solid solution of Cr and Fe. As shown in FIG. 2, the average reflectivity in the range of the wavelengths of 620 to 750 nm is 25% or less, but since the pigment is a black pigment, it is difficult to transmit lights in the visible region, in particular, lights in the wavelength of 620 to 750 nm exhibiting red color. Therefore, when the black pigment described in Patent Literature 5 is used particularly in a red paint, it is difficult to obtain high highlight, and designability of a coating film product is impaired. Further, in Patent Literature 5, the black pigment is manufactured by heat treatment at 800 to 1400° C., and in such condition particles usually tend to coarsen. The coating film using such particles tend to have lower transmittance and higher haze value, which may impair designability of a coating film product.
Patent Literature 6 filed by the present applicant discloses a method of producing various nanoparticles of an iron oxide and the like between two processing surfaces being capable of approaching to and separating from each other and rotating relative to each other. However, the iron oxide nanoparticles described in Patent Literature 6 are the nanoparticles of black iron oxide (Fe3O4: magnetite) and yellow iron oxide (FeOOH: goethite), and it was not observed that these iron oxide nanoparticles have ultraviolet ray protection ability, or properties to transmit or reflect a visible light, especially a red light. Further, in the first place, suppression of the specific characteristics expressing in oxide particles themselves was not described in Patent Literature 6, and thus, color characteristics of oxide particles themselves was not investigated sufficiently so far. Therefore, a composition for a laminated coating film is desired which can be suitably used in both aspects of ultraviolet ray protection ability and designability.